A variety of polymers of brominated styrene are known to the art. These brominated polystyrenes are commonly used as flame retardant additives, and are produced by one of two basic methods--the bromination of polystyrene or the polymerization of bromostyrene monomers.
In general, the materials made by the two production methods are not equivalent. For example, bromination of polystyrene will result in undesirable side chain halogenation, causing a reduction in thermal stability or requiring expensive treatment to remove the more labile bromine atoms. Polymers prepared by the polymerization of bromostyrene do not have undesirable side chain halogenation, and are preferred for their relatively greater thermal stability.
Not only do the two methods of preparing brominated polystyrenes provide different end products, there are also numerous disadvantages inherent to the bromination of polystyrene approach. First, such methods require that the polymer be solubilized, necessitating isolation and purification procedures that may add significantly to production costs. Also, because the product is recovered from solution, the final product will be a dusty powder unless some type of compaction step is included at additional cost. Similarly, unless a post-production compounding step is used, the introduction of co-additives is limited to dry blending with other powders.
A more significant disadvantage of the bromination of polystyrene method is that the brominated polystyrenes produced are limited to copolymer compositions and molecular weights that are readily available. In addition, the products must be structures that are stable to, and will not interfere with, the bromination process.
The polymerization of bromostyrene has several advantages over the bromination of polystyrene. As mentioned, it provides a more thermally stable product because side chain halogenation is avoided. Also, this method can be used to produce a continuum of molecular weights and bromine contents not otherwise available. Further, the polymerization can be accomplished without the use of solvents, and is readily adaptable to more economical continuous production processes. In addition, a broad spectrum of copolymer compositions may be produced simply by adjusting the monomer feed. Finally, production of convenient non-dusting pellets (with the option of incorporating auxiliary additives) is a natural by-product of the inventive polymerization process, and may be provided at no additional cost.
Nowhere in the literature of bromostyrene polymers is there any indication that the practical bulk polymerization of these monomers to produce a highly brominated compositions has been addressed. One reason for this omission may be the lack of thermal stability of the brominated materials. Conditions that would normally be used to prepare commercial polystyrene (such as flash devolatilization at temperatures approaching 300.degree. C.) would cause thermal breakdown of most brominated materials, and product discoloration and equipment corrosion would result. Processes using solution and emulsion techniques avoid any possibility of decomposition, even though they are at an economic disadvantage.
A need therefore exists for improved methods of continuously polymerizing bromostyrenes. In particular, a need exists for a method of polymerizing bromostyrenes without the need of solvents and their associated disadvantages. A need also exists for a method of polymerizing bromostyrenes in which the reaction is run to a high degree of completion in a relatively brief period of time. The present invention addresses these needs.